Update:

I FINALLY got around to doing this and it works perfect. My front speakers are hooked up to an amp in the back, and I "jumped" the front speaker wires coming out of the car-side of the harness to the rear speaker wires.

Now, the only thing that is hooked up to the rear speakers is onstar. I tested and everything works perfect. (I guess I'm lucky and got a working GMOS-LAN-03!)

 

Good to hear you where able to get it to work. I figured you would. So your not using the rear door speakers with your HU? Just the chimes?

 

Exactly, I'm only worried about the front speakers, wasn't ever considering amping the rears...so they're perfect for onstar and chimes.

the only thing is that I'm going to have to remember what I did when I trade in the car so I can "unjump" all the wires when I put the factory radio back in.

The onstar and chimes are a decent volume as well, I was a little worried that I wouldn't be able to hear onstar too well from the rears, but there is no issue at all.